The present disclosure relates to a Hall Effect sensor for measuring current in medium-voltage applications. In certain embodiments, the present disclosure relates to a Hall Effect sensor for measuring current flowing from a medium-voltage variable-frequency drive to a load motor.
Power supplies configured to control a flow of energy between a first alternating current (AC) system and a second AC system are used in a variety of commercial and industrial applications. For example, a power supply is typically used in AC motor control and operation systems. Various power supplies convert energy from a first frequency and voltage to a second frequency and voltage. The first and/or the second frequency may be variable, for example from −330 Hertz to +330 Hertz, and may include a frequency of 0 Hertz, or DC.
In most power supplies, it is necessary to include devices to measure large currents in the first and/or the second AC system. These devices produce a small-scale signal which replicates the behavior of the large measured current, but is galvanically isolated from it. This isolated signal is then used for various purposes in the control circuits of the power supply. Often the current to be measured is the output current of the power supply, so that the sensor is located just before the output terminals.
For applications in which the current to be measured has a fixed frequency, or a frequency that varies over a limited range, a current transformer can often be used as the measuring device. However, standard current transformers generally cannot be used below a frequency of 25 Hertz. Custom-designed current transformers may extend the lower limit somewhat, but current transformers cannot be used for 0 Hertz, or DC.
Therefore in power supplies with a wide range of the first and/or the second frequency, it is customary to use current sensors based on Hall Effect technology instead of current transformers, Numerous suppliers offer Hall Effect current sensors for low-voltage applications. The principal market for these low-voltage sensors are variable frequency drives for AC motors operating at 1000 volts and below. This range is here designated as “low-voltage”.
There is also a growing market for variable frequency drives operating above 1000 volts, typically in the range from 1000 to 69,000 volts, This range is here designated as “medium-voltage”, Most commercially available Hall Effect current sensors are not sufficiently insulated for medium-voltage. A few Hail Effect current sensors are available with very large apertures, which can achieve higher levels of insulation by means of large air spacings. However, such large-aperture sensors are bulky and costly. Another disadvantage is that air tends to break down in an electric field much weaker than can be supported by many solid dielectric materials, and the break-down strength of air becomes even weaker as altitude is increased.
For these reasons, manufacturers of medium-voltage power supplies typically use conventional low-voltage Hall Effect current sensors. Because the sensor does not have sufficient insulation, a shielded medium-voltage cable is used to pass current through the low-voltage sensor. The shielded cable confines the electric field inside the cable insulating material, so that large external air spacings are not needed. However, where the shielded cable is terminated, bulky stress-cones are required. If the shielded cable is carrying the output current from the power supply, a separate termination device must be provided to receive both the shielded cable and also the user's load cables, Another drawback is that safety codes require a metal barrier between medium-voltage and low-voltage circuits, so that it is necessary to surround the low-voltage sensor with a grounded metal barrier box. Shielded cables for medium-voltage have much larger diameter than low-voltage cables of the same current capacity. Typically the largest shielded cable that will pass through the aperture of a low-voltage Hall Effect current sensor can handle only half of the current that the sensor can handle. For higher currents, two cables and a second sensor must be used, even though the first sensor alone could handle the current. These measures generally cost much more than the first Hall Effect sensor alone, and occupy a lot of space in the power supply enclosure.
FIG. 1 shows a prior art example to illustrate the drawbacks of using low-voltage Hall Effect current sensors in a medium-voltage power supply with an output current sensing circuit 100. The circuit 100 includes a metal box 102 as a barrier between the low-voltage sensing components and the medium-voltage drive components. A pair of shielded cables 104a and 104b pass through the barrier box 102, with grommets 103a, 103b, 103c, and 103d to protect the cable from the metal edges. Each of the shielded cables 104a and 104b also pass through low-voltage Hall Effect sensors 108a and 108b respectively, mounted inside the barrier box 102. The shielded cables 104a and 104b may include various insulations such as cross-linked polyethylene (XLPE). Stress cones 106a, 106b, 106c, and 106d must be installed to avoid a concentration of the electric field at the ends of the cable shields. The shields in the cables 104a and 104b must be grounded as shown at 112a. The shields in the stress cones 106a, 106b, 106c, and 106d must be grounded as shown at 112a and 112b. The barrier box 102 must be grounded as shown at 112b. 
In FIG. 1 each of the shielded cables 104a and 104b are electrically connected to a medium-voltage bus bar 116 such that the high-current produced by the power supply is divided into two paths through the circuit 100. The bus bar 116 must be supported by insulating standoffs 120c and 120d. After passing through the low-voltage Hall Effect sensors 108a and 108b, and the barrier box 102, each shielded cable 104a and 104b is further electrically connected to a second bus bar 118, where cables for delivering the power to the load can be operably connected by the user. The bus bar 118 must be supported by insulating standoffs 120a and 120b. 
Within barrier box 102, the Hall Effect sensors 108a and 108b are operably connected to tow-voltage control wires 110a and 110b respectively. The control wires 110a and 110b provide control power to the sensors 108a and 108b, and transfer any signals generated by the Hall Effect sensors 108a and 108b to the control circuits of the power supply. The control circuits may add the signals from sensors 108a and 108b to obtain a replica of the original current in the bus bar 116. To provide a barrier between the low-voltage control wires 110a and 110b and the medium-voltage circuits, a rigid or flexible metal conduit 114 is typically used.
In a typical motor drive or power supply, the cost of the second Hall Effect sensor, the shielded cables, the stress cones, the barrier box, the standoffs, the miscellaneous hardware, and the assembly labor can be many times greater than the cost of the first Hall Effect sensor alone. Also the volume required for the complete assembly is many times greater than the volume of the first Hall Effect sensor alone. For a multi-phase motor drive, the high-current sensing circuit 100 as shown in FIG. 1, or a similar current sensing circuit, may be duplicated for each phase, thereby further increasing the cost and size. The total volume required for all the current sensing circuits can have a significant impact on the overall size of the drive.